In general, lithography refers to processes for pattern transfer between various media. A lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Typically, a "transparency" of the subject pattern is made having areas which are selectively transparent, opaque, reflective, or non-reflective to the "projecting" radiation. Exposure of the coating through the transparency causes the image area to become selectively crosslinked and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble (i.e., uncrosslinked) areas are removed in the developing process to leave the pattern image in the coating as less soluble crosslinked polymer.
Projection lithography is a powerful and essential tool for microelectronics processing. As feature sizes are driven smaller and smaller, optical systems are approaching their limits caused by the wavelengths of the optical radiation. "Long" or "soft" x-rays (a.k.a. Extreme UV) (wavelength range of .lambda.=100 to 200 .ANG.("Angstrom")) are now at the forefront of research in efforts to achieve the smaller desired feature sizes. Soft x-ray radiation, however, has its own problems. The complicated and precise optical lens systems used in conventional projection lithography do not work well for a variety of reasons. Chief among them is the fact that there are no transparent, non-absorbing lens materials for soft x-rays and most x-ray reflectors have efficiencies of only about 70%, which in itself dictates very simple beam guiding optics with very few surfaces.
One approach has been to develop cameras that use only a few surfaces and can image with acuity (i.e., sharpness of sense perception) only along a narrow arc or ringfield. Such cameras then scan a reflective mask across the ringfield and translate the image onto a scanned wafer for processing. Although cameras have been designed for ringfield scanning, e.g., Jewell et al., U.S. Pat. No. 5,315,629 and Offner, U.S. Pat. No. 3,748,015, available condensers that can efficiently couple the light from a synchrotron source to the ringfield required by this type of camera have not been fully explored. Furthermore, full field imaging, as opposed to ringfield imaging, requires severely aspheric mirrors in the camera. Such mirrors cannot be manufactured to the necessary tolerances with present technology for use at the required wavelengths.
The present state-of-the-art for Very Large Scale Integration ("VLSI") involves chips with circuitry built to design rules of 0.25 .mu.m. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet ("UV") delineating radiation. "Deep UV" (wavelength range of .lambda.=0.3 .mu.m to 0.1 .mu.m), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 .mu.m or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths.
Two x-ray radiation sources are under consideration. One source, a plasma x-ray source, depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet ("YAG") laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 .mu.m to 250 .mu.m spot, thereby heating a source material to, for example, 250,000.degree. C., to emit x-ray radiation from the resulting plasma. Plasma sources are compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). Another source, the electron storage ring synchrotron, has been used for many years and is at an advanced stage of development. Synchrotrons are particularly promising sources of x-rays for lithography because they provide very stable and defined sources of x-rays.
A variety of x-ray patterning approaches are under study. Probably the most developed form of x-ray lithography is proximity printing. In proximity printing, object: image size ratio is necessarily limited to a 1:1 ratio and is produced much in the manner of photographic contact printing. A fine-membrane mask is maintained at one or a few microns spacing from the wafer (i.e., out of contact with the wafer, thus, the term "proximity"), which lessens the likelihood of mask damage but does not eliminate it. Making perfect masks on a fragile membrane continues to be a major problem. Necessary absence of optics in-between the mask and the wafer necessitates a high level of parallelism (or collimation) in the incident radiation. X-ray radiation of wavelength .lambda..ltoreq.16 .ANG. is required for 0.25 .mu.m or smaller patterning to limit diffraction at feature edges on the mask.
Projection lithography has natural advantages over proximity printing. One advantage is that the likelihood of mask damage is reduced, which reduces the cost of the now larger-feature mask. Imaging or camera optics in-between the mask and the wafer compensate for edge scattering and, so, permit use of longer wavelength radiation. Use of extreme ultra-violet radiation (a.k.a., soft x-rays) increases the permitted angle of incidence for glancing-angle optics. The resulting system is known as extreme UV ("EUVL") lithography (a.k.a., soft x-ray projection lithography ("SXPL")).
A favored form of EUVL is ringfield scanning. All ringfield optical forms are based on radial dependence of aberration and use the technique of balancing low order aberrations, i.e., third order aberrations, with higher order aberrations to create long, narrow illumination fields or annular regions of correction away from the optical axis of the system (regions of constant radius, rotationally symmetric with respect to the axis). Consequently, the shape of the corrected region is an arcuate or curved strip rather than a straight strip. The arcuate strip is a segment of the circular ring with its center of revolution at the optic axis of the camera. See FIG. 4 of U.S. Pat. No. 5,315,629 for an exemplary schematic representation of an arcuate slit defined by width, W, and length, L, and depicted as a portion of a ringfield defined by radial dimension, R, spanning the distance from an optic axis and the center of the arcuate slit. The strip width is a function of the smallest feature to be printed with increasing residual astigmatism, distortion, and Petzval curvature at distances greater or smaller than the design radius being of greater consequence for greater resolution. Use of such an arcuate field allows minimization of radially-dependent image aberrations in the image. Use of object:image size reduction of, for example, 5:1 reduction, results in significant cost reduction of the, now, enlarged-feature mask.
It is expected that effort toward adaptation of electron storage ring synchrotron sources for EUVL will continue. Economical high-throughput fabrication of 0.25 .mu.m or smaller design-rule devices is made possible by use of synchrotron-derived x-ray delineating radiation. Large angle collection over at least 100 mrad will be important for device fabrication. Design of collection and processing optics for the condenser is complicated by the severe mismatch between the synchrotron light emission pattern and that of the ringfield scan line.
Aside from the quality of the optics that are employed in EUVL systems, factors that influence the quality of the printed wafers fabricated include the ability of the systems to prevent contaminants from depositing onto the surfaces of lens and mirrors and other optical devices. A possible source of contaminants are the hydrocarbons generated by the wafer upon exposure to radiation. Reducing the amount of such deposits will enhance overall quality and performance. Another factor that will affect the quality of the printed wafer is the ability of projection photolithography systems to be vibrationally isolated.